The present invention relates in general to semiconductor integrated circuits and, more particularly, to semiconductor integrated circuits which are manufactured such that they can be made to have the logic functions desired by customers.
The technology of semiconductor elements has been strikingly developed since the invention of the transistor by Dr. William Shockley and others at the Bell Laboratories in the U.S.A in 1948. The integrated circuits (ICs) which have a number of active elements or logic gates formed on one chip, have been revolutionized in the degree of integration, in operational speed and in their reliability, along with the rapid advancement in manufacturing techniques. By way of example, LSIs and VLSIs have been realized in which large-scale high-performance electronic circuit systems have been integrated on a square chip having a side with a dimension of only several millimeters.
In order to realize an LSI in which a desired electronic circuit such as a logic function circuit is integrated, there are basically two different approaches. The first approach is a custom LSI, in which an optimum exclusive logic circuit conforming to a desired design specification is realized on a semiconductor chip. To fabricate a custom LSI, CAD (computer-aided disign) techniques for designing an exclusive pattern are introduced in the circuit design. Thus, a functional chip which can operate at a high speed and has a high degree of integration can be produced. On the minus side, however, the custom LSI requires a long time to development, and it is difficult to change a specification once it is decided. Further, there is no guarantee that mass production is possible, which leads to a high risk on the part of the user as far as cost is concerned. Semi-custom-made LSIs are also well-known in the art, but they are not expected to totally overcome the deficiencies noted above. The second approach for realizing an LSI having a desired logic function is a microprocessor (or microcomputer). This approach integrates the usual computer function on one chip. The microprocessor thus can fulfill a wide variety of functions as desired under software program control. Thus, the same microchip can be used for a variety of different applications. This means that a microprocessor can meet the requirements to obtain a large number of different LSIs, each of which is manufactured only in small quantities, at low cost and in a comparatively short period of time. A disadvantage of this microprocessor is that its operational speed is far below that of the custom LSI noted above because the internal signal transfer is controlled by software. Therefore, the microprocessor is inevitably powerless, particularly when it is applied to a system where high operational speed is required.
Up to now, there has been no semiconductor IC or LSI which can meet the need for a variety of exclusive LSIs which can be manufactured only in small quantities by readily realizing desired functions and which are also superior in performance, particularly in operational speed. Of course, other approaches derived from the two basic approaches described above (e.g., the gate-array LSI, etc.) exist and are being advanced, but none of them can simultaneously meet the two requirements noted above.